To achieve the concise interpolation for spatial curves, the strategy of cross-octant interpolation was researched based on open-architecture CNC system. The sub-modules of the interpolation module for complete software type CNC system or NC embedded PC type system were designed. Take cylinder spiral-line and spatial line for instance, some studies that include principles and techniques of affine transformation, automatic cross-octant, and calling basic sub-modules of interpolation were carried out. The interpolation module for open-architecture CNC system can be divided into some sub-modules to develop. Thus, the balance between the openness and the real-time of the system are achieved. Some strategies and principles in this paper can be also applied to other spatial curves.